Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to a soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed electric discharge produced plasma (“DPP”), the plasma may be produced by an electrical discharge between a pair of electrodes. In another method, the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. This later process is referred to as laser produced plasma (“LPP”).
For each of these processes, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps of source material that is not fully ionized in the plasma formation process.
These plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and grazing angle incident mirrors, the surfaces of metrology detectors, windows used to image the plasma formation process, and in the case of LPP, the laser input window. The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. In addition, some optical elements, e.g., the laser input window, form a part of the vacuum chamber and are thus placed under a stress when a vacuum is present in the plasma chamber. For these elements, deposits and heat can combine to fracture (i.e., crack) the element resulting in a loss of vacuum and requiring a costly repair.
Accessing contaminated or damaged optical elements in the plasma chamber for the purpose of cleaning or replacing the elements can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.
With the above in mind, Applicants disclose systems and methods for reducing the influence of plasma-generated debris on the internal components of an EUV light source.